The present invention relates generally to the field of wireless communications and in particular to a method of reverse link load estimation using a reference signal.
Modern cellular wireless communication networks provide voice and data communication services to mobile stations throughout a geographic area, such as a cell or sector. Voice and data are transmitted on dedicated channels and on shared packet data channels from base stations to mobile stations within their coverage area. This is known as the forward link. Voice and data are also transmitted from mobile stations to one or more base stations, known as the reverse link.
For efficient and stable operation, it is imperative that the base station maintain an accurate estimate of the reverse link load. As used herein, the “load” is a measure of how congested, or how close to full capacity, a network, cell, or sector is at any given time. More specifically, the load is defined as the ratio of total received power at the base station to the thermal noise power. The total received signal power includes all received signals from mobile stations reaching the base station and the thermal noise. When this ratio is small, the load is defined as being low. Conversely, when the ratio is high, the load is defined as high. The reverse link load ratio is also known in the art as the rise-over-thermal (ROT).
Accurate estimation of the reverse link load is important for several reasons. First, it is used to help maintain system stability. For example, if the reverse link load becomes high, congestion control may be initiated to reduce the load. Second, knowledge of the reverse link load may help the wireless network run more efficiently at a finer time scale level (e.g., 800 Hz). For example, a sector with a low reverse link load over several 1.25 millisecond periods may allow traffic channels such as the reverse packet data channel (R-PDCH) to broadcast at higher rates, thus increasing the load. Third, the level of an “uncertainty buffer”—the back-off from a threshold indicating a highly loaded condition—may be reduced. The uncertainty buffer level must be increased as the load estimation accuracy decreases. Fourth, accurate reverse link load estimation may assist admission control and load balancing. These benefits are similar to improved system stability and efficiency, but at a larger time scale. For example, if the reverse link load of one carrier is high, with a highly accurate estimation, an incoming call request from a user may be diverted to an alternate carrier.
Current methods of estimating the reverse link load suffer from unacceptable inaccuracies. In one prior art technique, received signal power is directly measured, such as at the output of an automated gain control (AGC) circuit in the base station. This gives an inaccurate estimate of the reverse link load, particularly with respect to temperature changes. In another prior art approach, the absolute sector received power estimate (RSSI over all mobile stations) is calculated. However, each mobile station's received signal strength is a poor indicator of the reverse link load. Still another prior art approach utilizes the received signal to interference noise ratio (SINR) information of demodulated signals. This provides an inaccurate measure of reverse link load since the SINR information of non-demodulated signals is necessarily taken into account. Another approach that yields an inaccurate estimate of the reverse link load is the target load sum: sum_k [SINR_k/(1+SINR_k)] using set points. Still another inaccurate prior art reverse link load estimate is the energy sum—using received SINR_k (or Eb/Nt).
Reverse link power control is well known in CDMA systems. The base station sends power control information to each mobile terminal in a cell or sector at the rate of 800 Hz in CDMA 2000, and 1500 Hz in WCDMA. If the base station can clearly decode the information from a mobile terminal, it directs the mobile terminal to reduce its transmit power on the reverse link. If the base station detects errors in the signal received from the mobile terminal, it directs the mobile terminal to increase its transmit power. The base station uses a quality indication from the mobile terminal to determine whether there are errors in the received signal, and therefore to determine how to control the reverse link power level for the mobile terminal.
Reverse link power control is related to the reverse link load. As the reverse link load increases, more interference exists on the reverse link, degrading the quality of signals received by the base station from each mobile terminal in the cell or sector. Consequently, the base station must direct each mobile terminal to increase its transmit power in order to be able to decode the reverse link signals.